Heretofore, the two principal methods to generate a signal at a precisely controlled frequency were: The direct method using mixers and filters to add different frequencies to achieve the desired frequency, and The indirect method using a Phase Locked Loop (PLL) with frequency dividers.
The direct method results in a complex implementation because of the complicated signal processing--there are many oscillators, mixers and filters. The indirect method's main drawback is the long settling time when switching to a new frequency. This is the time required for the PLL to lock again at the new frequency.
The frequency of oscillators known in the art is controlled by an analog signal, resulting in a phase and frequency noise because of unavoidable noise in the analog control voltage.